(a) Field of the Invention
The present invention relates to a quantized precoder codebook optimizing device and method using a steepest descent algorithm. More particularly, the present invention relates to a quantized precoder codebook optimizing device and method for minimizing a maximum correlation between codewords of a codebook including quantized channels by using a steepest descent algorithm in a mobile communication system.
(b) Description of the Related Art
A conventional mobile communication system may not obtain instantaneous downlink channel information at a transmitter since downlink and uplink channel are different. Accordingly, it is required to provide feedback of channel information estimated by a receiving terminal to a transmitter to obtain the channel information at the transmitter. However, since the instantaneous channel information may not be transmitted in a situation in which a band of a feedback channel is limited, it is required to use an information compression method for reducing the channel information, and a quantized preceding method for precoding a symbol at a transmitting terminal by using quantized channel information has been suggested.
In the quantized precoding method, a space formed by channels with multiple antennas is uniformly quantized to generate a codebook, the codebook is shared by transmitting and receiving terminals, and a codebook index of a relatively small size is transmitted from the receiving terminal to the transmitting terminal rather than transmitting information of all channels. In this case, when the size of the codebook is large enough, performance is close to that of transmit maximum ratio combining (Transmit-MRC or MRT) and eigen beamforming method.
The codebook of conventional quantized precoder is designed so that codewords are distributed uniformly in a space formed by the channel, but it is not completely optimized since it has a limit in designing the codebook. It is determined by a correlation between the codewords.
A 3-bit vector codebook, a 6-bit vector codebook, and a 6-bit matrix codebook among conventional codebook design methods will be exemplified.
Firstly, the space formed by a vector channel is uniformly quantized by 8 vectors to form the 3-bit vector codebook, and the number of combinations of the generated codewords is small compared to other conditions (e.g., the 6-bit vector codebook and the matrix codebook). Accordingly, in the 802.16e, the 3-bit vector codebook is generated by a random full search method. The codebook of 2 or 3 transmit antennas is optimized by the random full search method. However, in a case of the codebook of 4 transmit antennas, since chordal distances between the codewords are not the same as each other, the channel may not be efficiently copied.
FIG. 1 shows a diagram representing chordal distance distributions according to the conventional 3-bit vector codebook when the number of transmitting antennas is 4 and the number of streams is 1. In FIG. 1, the chordal distances between the codewords are not the same.
In the case of the 6-bit vector codebook, a codeword is generated by codebook generation matrix and unitary transformation matrix. A frequency of codebook generation matrix and a householder vector of unitary transformation matrix are determined so that the codebook has the lowest maximum correlation (or the highest minimum chordal distance) between the codewords.
FIG. 2 shows a diagram representing chordal distance distributions according to the conventional 6-bit vector codebook when the number of transmit antennas is 3 and the number of streams is 1. In FIG. 2, the minimum chordal distance is not great enough since a searching operation is performed in a limited space.
FIG. 3 shows a diagram representing chordal distance distributions according to the conventional 6-bit matrix codebook when the number of transmit antennas is 4 and the number of streams is 2. In FIG. 3, the minimum chordal distance is not great enough since the codebook is extended from pre-designed vector codebook by performing a householder concatenation operation and a householder extension operation.
As described, the codebook of conventional quantized precoder is designed so that the space formed by the channel is uniformly copied, but it is not completely optimized since it has a limit in designing the codebook.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.